As a conventional smoke detector which detects the type of smoke, there is a smoke detector which uses a light projection device which emits light beams of a relatively long wavelength, and another light projection device which emits light beams of a relatively short wavelength.
In this type of smoke detector, the above respective light projection devices alternately emit light beams at different timings, a light receiving device receives scattered light generated due to the light beams from these light projection devices impinging on smoke particles, light reception signals output from this light receiving device are obtained separately for each light projection device, and further, a calculation is performed based on these light reception signals, to thereby determine the type of the smoke.